This invention relates to improvements in electric vehicles, such as locomotives for trains, and in particular to a method of reducing the inrush current that occurs when a transformer in such a vehicle is first connected to an alternating current supply. It also relates to a controller that is adapted to control the operation of such an electric vehicle.
It is well known to provide electric locomotives for trains with at least one electric motor that drives wheels of the locomotive. In many systems, power for the electric motor or motors is provided by connecting the motors to an overhead wire which runs along the length of the track. The wire is connected to a trackside power source such as a generator and is supported from a catenary. The top of the locomotive is provided with a pantograph mechanism that in use pushes against the wire to provide an electrical connection between the wire (known as a trolley wire) and the electric motor. The motor draws a current from the trolley wire as required.
It has been proposed to supply electric locomotives with an alternating current supply at an alternating voltage through the trolley wire. A transformer is provided on board the locomotive which steps-down the alternating supply from the relatively high voltage needed for transmission along the trolley wire to a lower voltage suitable for supply to the motor and other electrical equipment on the train.
A particular problem occurs with an alternating current scheme when the transformer is initially connected to the trolley wire. For example, this occurs whenever the locomotive is about to start moving or perhaps after a period of constant descent. The transformer can draw a large asymmetrical current when it is initially connected to the trolley wire that can take several seconds to die away each time the transformer is connected to the supply. The DC content of this transient current and its duration can cause a voltage drop in the running rail used for the return of current from the train and this DC current can upset circuitry associated with the track.
The phenomenon of current inrush can better be understood by referring to FIGS. 4(a) and (b) of the accompanying drawings. In FIG. 4(a) the transformer is initially at a state of discharge and is connected to the voltage supply as the voltage crosses a peak. The resulting flux in the transformer changes sinusoidally at quadrature to the voltage. The transformer is usually designed so that the peak flux in this case does not quite saturate the transformer core.
In FIG. 4(b) it is assumed that the voltage supply is connected to the transformer as it crosses through zero volts. In this case, the flux rises rapidly to a peak value far in excess of that required to saturate the transformer. It can then be many cycles before the flux gradually returns to the correct steady state level. It is during this saturation period that a large current is drawn by the transformer.
Much of the signalling equipment on AC railway systems uses DC relay track circuits for train detection. These track circuits are particularly sensitive to transients, but other types of track circuit also tend to be affected. The DC track circuit works by introducing a small DC voltage between the running rails at one end section of a track, and detecting its presence by a relay at the other end. The presence of a train is detected when the wheel-sets short across the rails causing the track circuit to de-energise. Transients at DC present at connection can cause false readings in these safety critical train circuits.
An object of the present inventions is to ameliorate the problems of inrush current on connection of the transformer of an electrically powered vehicle to an electrical supply.
In accordance with a first aspect, the invention provides a method of controlling connection of a transformer of an electric vehicle to an alternating voltage supply carried by a trolley wire, the vehicle having a contact pad for electrically connecting the transformer to the trolley wire, the contact pad being moveable between a first position in which the contact pad is held away from electrical contact with the trolley wire and a second position in which the contact pad makes electrical contact with the trolley wire, the method comprising the step of judiciously controlling movement of the contact pad towards the trolley wire such that the air gap between the contact pad and trolley wire is broken down to connect the transformer to the trolley wire when the voltage present on the trolley wire is at or near its peak.
By arranging for the transformer to commence conducting at or near the peak voltage, inrush is controlled. This method substantially reduces the maximum level of the magnetizing inrush current and can eliminate the inrush entirely provided that there is no residual flux in the transformer when conduction starts. It is preferred that conduction commences at the exact peak.
As is normal in such electric vehicle power systems, the contact pad is carried on a pantograph and movement of the contact pad is achieved by moving the pantograph between lowered and raised positions.
A switching means is preferably provided between the contact pad and the transformer. The switching means is switchable between a closed state in which current can flow from the contact pad to the transformer and an open state in which the contact pad is isolated from the transformer, the method including the step of ensuring the switching means is closed before moving the contact pad towards the trolley wire. The switching means may be held in the open position when the contact pad is in the first position.
The method may further include the steps of
measuring the voltage present on the trolley wire when the contact pad is in the second position, and
if the measured voltage indicates that no supply is present on the trolley wire, re-opening the switching means to isolate the transformer from the trolley wire.
The method may also include the step of re-measuring the voltage on the trolley wire after the switch is re-opened to detect presence of a supply to the trolley wire. This re-measuring may be continuous or performed periodically.
The voltage may be measured directly using a voltage measuring circuit or indirectly, for instance by detecting whether a current is being drawn from the wire by the train.
In the event that a supply is detected while the contact pad is in its second position, the method may include the steps of sequentially moving the contact pad to the first position, closing the switching means and then moving the contact pad back to the second position.
The method may comprise moving the pantograph from the first to the second position of a speed of substantially 0.5 m/s, or less than 0.5 m/s (e.g., 0.3 m/s). Of course, it will be appreciated that the speed needed to cause breakdown at the peak voltage will depend on the voltage used, materials used, size of components, etc., and frequency used.
In accordance with a second aspect the invention provides a power system for an electric vehicle of the kind receiving AC power through a trolley wire, comprising an electric motor, a transformer for supplying power to the electric motor, a contact pad for electrically connecting the transformer to the trolley wire, means for moving the contact pad at a judiciously controlled rate from a first position in which the contact pad is held away from electrical contact with the trolley wire to a second position in which the contact pad makes electrical contact with the trolley wire, an isolating switch switchable between a closed state in which the transformer is connected to the contact pad and an open state in which the transformer is isolated from the contact pad, and control means for executing logical control of the position of the contact pad in conjunction with the state of the switch such that the switch is actuated to the closed state before the contact pad moves from the first position to the second position, whereby the transformer is connected to the supply on the trolley wire as the supply voltage reaches a peak due to the breakdown of the air gap between the trolley wire and the contact pad.
The contact pad is part of a pantograph and the control means is connected to control a pantograph raising/lowering mechanism for moving the contact pad between the first and second positions.
The power system may further comprise a user input device for generating user request signals to the control means thereby to move the contact pad between the first and second positions in response to a user input. The user input signals may be supplied by a driver of the train. One input signal may comprise a xe2x80x9cconnectxe2x80x9d signal upon receipt of which the controller is adapted to connect the transformer to the wire in the aforementioned manner. The other may comprise a disconnect signal upon which the controller is adapted to disconnect the transformer from the wire.
The controller may be adapted to disconnect the transformer from the supply by opening the isolating switch and subsequently lowering the pantograph.
The power system may further comprise sensor means for outputting a signal to the control means indicating the presence or absence of a supply voltage on the trolley wire when the contact pad is in the second position, the control means being adapted to output a signal to open the switch to isolate the transformer from the trolley wire if the sensor means indicates the absence of a supply voltage. The sensor measures the voltage present at a point between the contact pad and the switch.
If the sensor means indicates that a supply voltage is present on the wire when the contact pad is in the second position and the switch is open, the control means is adapted to output signals to sequentially lower the pantograph, close the switch and then move the contact pad back to the second position.
The controller may be adapted to periodically detect the presence of a supply when the pantograph is raised.
According to a third aspect, the invention provides an electric vehicle incorporating a power system in accordance with the second aspect of the invention.